Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

分析：

使用回溯，用temp记录路径。当遍历到叶子结点时，一个结果就生成了。

Java：

class Solution {
    List<String> res = new ArrayList<>();
    StringBuilder temp = new StringBuilder();
    String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    void dfs(int index, String digits) {
        if (index == digits.length()) {
            res.add(temp.toString());
            return;
        }
        int num = digits.charAt(index) - '0';
        for (int i = 0; i < map[num].length(); i++) {
            temp.append(map[num].charAt(i));
            dfs(index + 1, digits);
            temp.deleteCharAt(temp.length() - 1);
        }
    }

    public List<String> letterCombinations(String digits) {
        if ("".equals(digits)) {
            return new ArrayList<>();
        }
        dfs(0, digits);
        return res;
    }
}